In my opinion, he does this because he wants to impress Daisy. He is literally looking for items to impress her with. I think too he probably wants every nook and cranny to be cleaned up before he lets her come over. Gatsby is this very cool man on the outside, but on the inside, I think he is very insecure about most things.

I do agree that the light that shines in the house is symbolic of the new light in his life. For Gatsby, everything was dark until this moment. This moment of reunification with Daisy means everything has turned positive and illuminated for him.

What Gatsby tells Nick is that he has the house lit up like this because he has been "glancing into" some of the rooms. So it seems that he has just been looking around and has left the lights on.

However, I think there is something more important going on here. Gatsby is looking forward to meeting Daisy again. Because of this, I think that having all the lights on is sort of symbolic. He is taking stock of his life, looking at what it is that he has to offer Daisy.

In The Great Gatsby, Gatsby tells Nick all of the lights are on in his mansion because he's been "glancing into some of the rooms."

From a distance in his taxi, Nick at first thinks there must be a fire. Then when he sees that the brightness is coming from Gatsby's house he assumes there's a party going on. When that turns out not to be the case, and Gatsby says he was just glancing into the rooms, Nick appears unsure of what to think.

The suggestion is, I believe, that the lights are all on so that Gatsby's house can be seen from Daisy's house. This is a reversal of Gatsby staring at the light on Daisy's dock. This is wishful thinking on Gatsby's part, of course--Daisy certainly isn't staring at his house from across the water.

There may also be a sense that Gatsby is inspecting the home in order to prepare for Daisy's eventual visit. Remember that one of the first things he does with Daisy once they do meet again, is to give her a tour of the house.